Extending the depth of focus of imaging systems is a very important technology allowing the system incorporation into various applications, including inter alia medically related applications where elements, such as cameras, are to be inserted into the body in order to observe and detect problematic tissues; as well as ophthalmic industry including glasses for spectacles, contact lenses, intraocular lenses or other lenses inserted surgically into the eye. The extended depth of focus solution is also needed for optical devices like microscopes or cameras for industrial, medical, surveillance or consumer applications, where focusing of light is required and where the conventional focusing techniques is based on the use of a multitude of lenses with the need of relative displacement between the focusing arrangement and an imager and/or object plane, by mechanical movement, either manually or electronically driven.
Various approaches have been proposed to achieve extended depth of focus. Some of the proposed techniques are based on the use of an aperture coding which later on require digital decoding (post-processing); some other approaches are based on the use of aperture apodization, e.g. by placing complicated diffractive optical elements or by all-optical means where a phase mask is added to the entrance pupil of the imaging lens. The polarization of light can add additional degree of freedom that may be used for the compromise done in the optimization process of the imaging system. Recently, a new approach has been developed in which a birefringent lens is used that produces two focal lengths (for each principle polarization state). By proper design of the lens the two focal lengths can be chosen such that the focusing range is extended (S. Sanyal, “High focal depth with a quasi-bifocus birefringent lens”, Appl. Opt., 39, 2321-2325 (2000)). However the fabrication of such a lens is complicated and expensive.